Many engine air cleaners are designed so that particulate matter separated from the air may be continuously removed or scavenged through a negative pressure connection to the cleaner, and it is known that a suitable negative pressure for this purpose can be obtained by inserting a venturi into the engine exhaust channel. U.S. Pat. No. 3,137,553 to Billey teaches an ejector arrangement of this sort. U.S. Pat. No. 3,419,892 to Wagner et al. teaches that the functions of muffling and ejection can be obtained in a single exhaust ejector unit, as will be discussed in more detail below. The Wagner structure is compact and efficient, but is somewhat expensive to manufacture and assemble, and also is not susceptible of design so that the intake for combustion gases is centered on the unit. This means that when the unit is mounted in its usual fashion, with its longitudinal axis vertical, the considerable weight of the unit is effectively off-axis, which makes for some mechanical instability and strain on the interconnections involved, under the conditions of vibration associated with the operation of such equipment, and may require accessory mounting brackets or braces in some installations. An alternative expedient of intricate internal structure known in the art, and as more fully described herein) supplies the combustion gases centrally at one end of a muffler-ejector structure having an ejector built centrally into the other end, the scavenging air being conducted through the muffler to the ejector at an off-center site.